This invention relates to an apparatus for soldering electronic parts and the like to a printed circuit board by the so-called dipping method, and more particularly to a soldering apparatus for applying solder uniformly on a flat soldering surface of the printed circuit board.
The conventional procedure for mounting electronic parts such as resistors and capacitors on a printed circuit board has been to insert lead wires of the electronic parts in the holes which are provided in the conductive portions of the printed circuit board and to dip them in molten solder to solder them in the respective positions. In such a case, the flat soldering face on the underside of the printed circuit board is maintained parallel with the surface of the molten solder during the dipping operation. Therefore, a great force is required at the instant of disengaging the soldering face from the surface of the molten solder, and unnecessary solder tends to remain on the underside of the printed circuit board, forming solder bridges or resulting in uneven finished surfaces. There has also been a problem of incomplete soldering due to the gases which are trapped between the soldering face and the surface of the molten solder when contacting them.
Problems are encountered also in soldering apparatus of the type where a printed circuit board is supported on a carrier which is circulated along a transfer path in the form of a closed loop and dipped in molten solder in a solder vessel which is provided at a predetermined position in the transfer path. As shown in FIG. 1, in this type of soldering apparatus, a printed circuit board 2 with parts to be soldered 1 is detachably held on a carrier 3 which has its front and rear wheels 4 and 5 mounted on a transfer rail 6, the carrier 3 being driven along the transfer rail 6 by a conveyor chain (not shown). The transfer rail 6 is gradually lowered by a relatively long downward slope II to have a flat sunken portion III over a solder vessel 7 and then raised to the initial transfer level by an upward slope IV which succeeds the flat sunken portion III. When the carrier 3 is on the flat sunken portion III, the underside of the printed circuit board 2 is dipped in the solder vessel 7 to solder the parts 1 to the board 2, thereafter lifting the carrier 3 to the level V along the upward slope IV. The carrier 3 is lowered and lifted in this manner to effect the soldering.
In this apparatus, in order to guide the printed circuit board 2 into the solder vessel, it is necessary to introduce the fore end of the printed circuit board 2 into the solder vessel 7 while its rear portions are still going down the slope II. Therefore, the sunken portion III must have a large distance L.sub.1 with long slopes II and IV of an easy grade. In addition, the solder vessel 7 is required to have a large length in the direction of travel of the carrier 3. As a result, there arise problems such as the increased size of the apparatus as a whole, the need for a larger amount of molten solder and for a greater amount of heat for maintaining the solder in molten state.